Mouse epidermal cell cultures have been utilized as a model for studying mechanisms of epithelial carcinogenesis in vitro. Untreated cells have a limited lifespan in culture while carcinogen treated cells from longterm lines, some of which are tumorigenic in animals. Carcinogens induce both excision repair and post-replication repair in these cultures and by use of a radioimmunoassay the fate of the carcinogen-nucleoside adduct formed in culture can be followed. Promoters stimulate ornithine decarboxylase activity and DNA synthesis after brief exposure. Exogenous polyamines enhance the proliferative response while steroids inhibit it. The steroid appears to act by decreasing the cell's ability to respond to polyamines and its effects can be overcome by exogenous polyamines. BIBLIOGRAHIC REFERENCES: Yuspa, S.H., Ben, T., Patterson, E., Michael, D., Elgjo, K., and Hennings, H.: Stimulated DNA synthesis in mouse epidermal cell cultures treated with 12-0-tetradecanoyl-phorbol-13-acetate. Cancer Res. 36: 4062-4068, 1976. Yuspa, S.H., Elgjo, K., Morse, M.A., and Wiebel, F.J.: Retinyl acetate modulation of cell growth kinetics and carcinogen-cellular interaction in mouse epidermal cell cultures. Chem. Biol. Interact. 16: 251-264, 1977.